Coatings on parts and products are used in industry for several different purposes. Certain coatings, for instance, are applied to surfaces to provide low friction, non-stick, abrasion resistant or other specific properties. When a harder object contacts a surface and friction is created, the friction of the force on the surface wears away a portion of that surface. Over time, the cumulative effect of repetitive or continuous contact between the object and the surface causes a substantial portion of the surface to wear away so that the surface is no longer functional. One example of such an object applying such a force is the force applied between metal cooking utensils (i.e., the objects) and cooking surfaces such as the non-stick coatings applied to surfaces of pots or pans. Continuous contact between the utensils and these cooking surfaces causes the coatings applied to the surfaces, such as non-stick coatings, to gradually wear away and diminish the effectiveness of the coatings. Therefore, manufacturers have developed abrasion-resistant, non-stick coatings incorporating harder and more durable particles, which resist the abrasive forces created by cooking utensils as the utensils contact the coated surface of the cooking surfaces. As a result, these coatings enhance the durability of cooking surfaces, however, the coatings compromise the non-stick properties of the surface. In addition, the introduced hard particles are not attached firmly to the surface to provide rigid support of the relatively soft non-stick material topcoat. The loose particles can therefore, dislodge from the surface relatively easily.
Wear also occurs, for example, when a surface or surfaces of a component are subject to continuous and prolonged forces such as rotational or reciprocating forces applied by a first object which basically, rubs against the surface of a second object, creating friction between the objects. The friction generally wears away one or both of the surfaces. If one surface is sacrificial in nature, commonly including softer particles or materials versus harder particles or materials, it will wear sooner than the surface including the harder, more durable surface. To prevent this, coatings, which are comprised of relatively soft materials, are applied to a part or parts so that the parts can be replaced easily when the coating wears out. An example is a PTFE-coated solenoid plunger inside a housing of a solenoid which must remain dry or without fluid lubrication in use. In operation, the PTFE-coated solenoid plunger wears at a predictable rate due to friction created between the surface of the plunger and the surface of the housing. Therefore, the plunger can be exchanged for a newly coated plunger on a predictable maintenance schedule. This is particularly appropriate when no fluid lubricants can be introduced into a system.
Another example of intended sacrificial wear is a wear end plate in a rotary gear pump. A thin layer of bronze is attached to, for example, a replaceable iron plate. This plate can be changed when the bronze layer wears.
Other methods of applying reinforcing coatings to surfaces to provide wear resistance for soft coatings have been developed. In another example, metal spray coatings, such as bronze, are arc sprayed or flame sprayed onto metal parts to produce an intermittent bronze coating, which is subsequently coated with PTFE and finished into a smooth surface. The wear resistance of the bronze particles reinforce the relatively weak strength of the PTFE, which has a very low friction however with low wear resistance. This bronze/PTFE coating system will wear longer than a unreinforced PTFE coating.
Coatings are also used for many applications such as for rust resistance, corrosion resistance and as functional or protective coatings. In one example, plastic electric motor brush covers are coated with coatings containing metal to absorb and conduct electrical emissions to reduce electrical emissions coming from electric fuel pumps in an automobile or other vehicle.
Coatings may be applied in various different known manners. One method of applying the coatings is to spray the coatings on to a substrate such as metallic material. The coatings are usually sprayed so that the coatings completely cover the surface or surfaces of the substrate that require the coating. Typically, a bonding agent such as primer is used to initially coat the substrate to promote the adhesion of the coating to the substrate. The bonding agent or primer is typically applied to a cleaned surface of the substrate. Additionally it has been known to add particles to the liquid primer and/or the base coating to increase the abrasion resistance of the liquid coating and primer. As a result, the coating or coatings applied to the substrate has greater abrasion resistance.
In one known coating process described in U.S. Pat. No. 5,492,769, hard particles are embedded in a surface layer on a substrate to improve the scratch or surface wear resistance of the substrate. The surface of the substrate is prepared and then a thin layer of discrete, hard particles are applied to the surface of the substrate. The surface of the substrate is then softened using heat or a solvent. The surface may also be softened prior to the application of the particles to the surface if needed. After the surface of the substrate is softened, the particles are embedded into the softened surface of the substrate using pressure. The particles are embedded in the surface so that approximately 50% of the volume of the particles are located at the surface layer. The softened surface is then allowed to re-harden, which embeds and bonds the particles to the surface of the substrate. This bonding method is generally limited to substrates such as plastics, polymers and softer metals, which are relatively softer than the particles being embedded in the surfaces of these substrates.
In another known coating process, solid particles are mixed with a liquid primer prior to applying the primer to the surface of the substrate. The particles are non-uniform or irregular in shape and the particles are also non-uniformly dispersed or mixed within the primer because the distribution of the particles cannot be controlled. Therefore, as the primer mixture is applied to the surface of the substrate, the unevenly distributed particles within the primer tend to pool or bunch up on particular areas of the substrate, particularly if the particles are very heavy. In addition, heavy introduced particles in a liquid coating system, will settle in different densities on horizontal, vertical or other pockets or depressions of a coated part. As a result, certain portions of the substrate include more particles than other portions of the substrate. This creates an uneven or non-uniform distribution of introduced particles on the substrate that has been coated. Thus, when a topcoat or final coat that contains heavy or dense particles is applied to the substrate, the topcoat or final coat also pools or builds up in the same areas on the substrate as the particles in the primer mixture. This diminishes the finish strength or quality of the surface of the finished product.
In a further known coating process, the particles are flame sprayed or applied in a molten droplet form and propelled onto the surface of the substrate. The molten particles adhere to the surface of the substrate. However, the molten particles are typically applied or sprayed onto the substrate at such a rate that the molten particles flatten or partially flatten as the particles hit or contact the surface of the substrate. Thus, the surface of such substrates are often uneven because of the varying shapes of the applied molten particles. As described above, this causes an uneven surface area or non-uniform surface area so that the coating that is subsequently applied to the particles is also uneven and non-uniform.
One known wear resistant coating sold under the trademark Excalibur® by Whitford Worldwide is formed using molten stainless steel particles and provides non-stick and initial low friction characteristics to surfaces of substrates but is limited in its application exclusively to metal substrates. This coating is applied to a substrate surface by first grit blasting the surface with a suitable abrasive to roughen it and promote the adherence of subsequent material layers. Then, stainless alloy particles are electric-arc sprayed onto the roughened metal surface of the substrate. The particles cool and harden forming varying surface configurations including high peaks and low peaks, which do not completely bond to the surface. Next, several layers of fluoropolymer coatings are applied to the particle layer and permanently bond all of the coatings in place on the surface of the substrate. The application of the molten particles, however, incurs the same problems as described above. Thus, the distribution of the size and shape of the molten particles in this process are generally not uniform or evenly distributed on the surface of the substrate.
The varying particle shapes in the above processes and coatings affect the uniform distribution of the particles on the surface of the substrate. The particles are applied to the surface of the substrate to increase the abrasion resistance of the subsequently applied coating. The irregularly shaped particles generally are not evenly or uniformly distributed on the surface of the substrate because the irregular surfaces of the particles generally do not correspond with each other. In addition, particle size may vary where some particles are much larger than other particles. Thus, some particles bunch together while other particles are spread apart based on the size and shape of the particles. This causes the particles to be non-uniformly distributed and be of non-uniform size on the surface of the substrate, which in turn, causes the coating applied to the particles to be non-uniform.
Furthermore, the irregular shape of the particles limits the consistency of the surface area created by the addition of the non-uniform particles to the substrate. As described above, irregularly shaped particles may bunch up or be spread apart on a percentage of the surface of the substrate and consist of varying heights. Therefore, the maximum surface area of each particle may not be exposed to prevent coating wear depending on whether the particles are partially or wholly covered by other particles and whether particles, such as molten metal particles, are deposited on top of previously deposited particles. Because the maximum exposable surface area of each particle is not consistently available, the total surface area generated by the addition of the particles to the substrate is limited or diminished. As a result, the bond between the coating and the particles is less consistent and weaker than if the maximum exposable surface area is exposed for coating by subsequent topcoats. This results in loose or non-adherent particles on top of the previously deposited particles.
Another reinforced metal/PTFE coating system consists of an electric arc spray of stainless steel or titanium particles subsequently coated with a two coat PTFE commercial non-stick coating. In one example, an aluminum cooking pan is grit blasted with aluminum oxide to roughen the aluminum surface to assist the mechanical attachment of the dissimilar metal stainless steel or titanium particles to the pan. The stainless steel particles are heated to provide molten stainless steel particles which are propelled toward the grit blasted surface. The deposited molten stainless steel or titanium particles cool and the result is a rough, varied group of various shaped metal particles which are mechanically attached to the rough grit blasted aluminum surface. This irregular surface, which resembles a mountain range of irregular shaped stainless steel particles, is then coated with two or more coatings of a commercial non-stick coating. When cooking utensils contact the reinforced non-stick coating, wear is reduced as the metal particles take the mechanical abrasion created by cooking utensils.
This stainless steel/PTFE coating system is exemplified by the Whitford Corporation's Excalibur® coating system. After the stainless particles are deposited on the aluminum, the Excalibur® coating system includes the steps of removing non-adherent particles of loose stainless steel from the aluminum. The loose, non-adherent or slightly adherent particles of stainless steel are removed by wire brushing, air cleaning or both prior to the application of the topcoats of PTFE based non-stick coatings.
Uneven or non-uniform distribution of a coating on a surface of a substrate causes several different problems or issues for manufacturers. An uneven coating on the surface of a particular part will cause uneven wear of the parts and subject the part to more friction if particles protrude through the final coating or topcoat. Conversely, if there are less reinforcing particles in an applied coating due to the non-uniform distribution of the reinforcing particles in the coating on the surface of the part, the coated surface of the part will have less wear resistance or wear protection. Thus, the greater friction and wear on the part may cause the part to fail during operation or not last as long in operation. Additionally, an uneven coating quality may cause a part to not perform as well in operation. An uneven surface roughness due to uneven internally distributed and dispersed reinforcing particles may also cause a part or parts requiring specific tolerance levels or dimensions to be outside acceptable limits and therefore not meet desired designed specifications, fail during assembly or during operation. If the coating over the irregular stainless particles is designed to provide non-stick or release properties, taller or larger stacked particles protruding through the coating quickly wear, dislodge easily and diminish the desired non-stick or release properties and cause sticking instead of preventing it.
In a mechanical wear situation, if the dispersed lubricant/reinforcing materials within the liquid coating, whether the materials are metal or non-metallic materials, are protruding, mechanical interference may be caused thereby preventing component motion. If the particles are not dispersed uniformly, the wear resistance and consistency of performance is diminished.
Additionally, liquid coating end use requirements for reinforcing a coating on a surface of a substrate to provide augmentation of, for instance, a soft coating to increase the wear resistance is presently accomplished by introducing hard particles to the liquid coating. The desired laminar placement of specific particles in the coating is impossible when particles are dispersed in a liquid coating. The placement of reinforcing particles within a coating in a specific area is not possible with a liquid coating containing dispersed particles. Furthermore, the placement of different wear resistant particles in a specific order or location with different properties cannot be done in a liquid coating application. Also, providing an extremely dense, near 100% dense layer of reinforcing particles under a smooth topcoat is not possible with a liquid coating. Even if multiple layers are applied, the possibility of particles moving and migrating in a liquid coating prevent assurance of and determination of density before subsequent coating layers are applied as the liquid carrier reduces the density of the interconnected particles. Additionally, if a liquid coating application includes heavy, hard particles such as certain abrasive particles, the coating is difficult to apply using conventional pumps and spray guns, which causes the pumps and spray guns to wear more rapidly.
Accordingly, there is a need for a coating method, and specifically a coating reinforcing underlayment and method of manufacturing same which enables a reinforced coating system to be evenly, uniformly and densely applied to the surface of a substrate.